The present invention concerns an antenna for radiofrequency (r.f.) transmission.
2. Discussion of Prior Art
The requirement for an antenna which provides transmission covering 360.degree. in azimuth is well known in, for example, combat identification systems where combat vehicles etc need to transmit a signal which allows them to be identified by friendly forces. The applicability of the current invention is not, however, restricted to this field and uses may be found in any situation where transmission covering an azimuth of 360.degree. is required, for example, in the area of local area networking where a number of peripheral devices may communicate by r.f. transmission rather than electrical or fibre optic link.
Several types of reflector antenna are known (see for example Kraus, J. D., Antennas, McGraw-Hill, 2nd Ed., 1988.). Conventionally, the reflector is used to direct or focus the energy into a narrow beam, but if the application requires an omnidirectional antenna pattern, then the reflector needs to spread the energy into a wide angle. This has been achieved using a dual reflector system using a parabolic subreflector (Orefice, M. & Pirinoli, P., "Dual reflector antenna with narrow broadside beam for omnidirectional coverage", Elec. Lett., Vol. 29, No. 25, Dec. 9, 1993, pp. 2158-2159.). If just a single reflector is preferred, one can use, for example, a beam having a fundamental Hermite-Gaussian radial intensity to illuminate a cone which reflects the radiation over 360.degree. in azimuth. However, a beam having such a radial intensity to illuminate has its maximum intensity illuminating the point of the cone and this causes scattering and interference which, in turn, causes high sidelobes and a ragged elevation pattern. Such a design is also difficult to model accurately.